Introduction
The fish farmers in western and southern Norway report that spiny dogfish often bite through the net and get in the fish cages. The holes in the cages can cause escapees of farmed fish leading to both financial loses and ecological challenge. The dogfish are usually attracted to dead fish found at the bottom of the cages. But inside the cage, they also eat and harm the live farmed fish causing welfare challenge. To prevent this , farmers continuously remove dead fish and must constantly inspect the cages with the help of divers and underwater cameras for any holes. Therefore , this is a financial, welfare and ecological challenge. To date there is no effective shark deterrent for fish farms. Hence, it is cru cial to test and develop an effective method to prevent spiny dogfish incidents in fish farms.
Materials and Methods
Here, we have tested active measures that acts through sensory system. Spiny dogfish of size 60-85cm were caught and housed in laboratory aquarium (flow through) of size 2m diameter and 85cm water height. It was supplied with sea water at 9 °C; the light intensity was 10 lux at 10cm above the water surface . Experimental tank was equipped with a custom-made low light camera. The electromagnetic (EM) pulse , sound of orca (natural predator) and skin extract from conspecific were used as deterrent stimuli; an extract form mackerel was prepared and used as an attractive stimulus. The animal behavior is recorded before and after application of stimuli and analyzed for change in locomotive behavior.
Results
The spiny dogfish showed change in locomotive behavior (increased/decreased speed) in response to EM, skin extract and food stimuli; however, it showed no change in response to sound of orca. Food stimulus or smell of dead mackerel induced food -seeking behavior- sharks were found probing the odour inlet. Both EM and skin extract induced avoidance response- sharks moved away from the source area.
Discussion
Shark barriers and deterrents have been developed against specific species of sharks; these have been used to keep sharks away from bathing areas, to offer personal protection for swimmers, divers and surfers; some of these have been tested to keep sharks away from bait and catch in line and net fishing. Their effectiveness varies depending on the species and geographical area; none of the m provide a full deterrence. Here, by evaluating the candidate shark-deterrents by quantifying the behavioral and physiological responses , we find that both EM and skin extract could be used as effective shark deterrent. Further field trials i n an affected production farms are necessary to evaluate the effectiveness of these methods from aquaculture installations.
Acknowledgement : Funding: Norwegian Seafood Research Fund, Ref.- 901704